Using in vitro mammalian cells (Chinese hamster V79 and CHO cells) irradiated with 260 KVp X rays, we will evaluate two hypotheses concerning the mechanisms of sensitization by oxygen (O2): Hypothesis (A): Oxygen sensitizes by reacting at cellular sites formed by reactions of hydroxyl radicals (OH), and Hypothesis (B): The superoxide anion radical (O2) plays a major role in cellular damage from ionizing radiation. To reach conclusions, we will observe how various treatments and chemical additives alter radiation sensitivity. (The steepness of a survival curve is taken as an index of radiation sensitivity.) A knowledge of the intracellular concentrations of added chemical agents is fundamental to this evaluation. These experiments are an essential step toward our long-range goal of selecting the most effective combination of sensitizers and/or protectors to use when the cells being treated are at different O2 concentrations. Such knowledge has immediate, obvious relevance to clinical oncology.